This invention relates to a method of, and the associated system for, rewriting data in a non-volatile memory, by which an interruption in power, even if it occurs during the rewriting of data, can be detected with ease when the power is restored.
In computerized numerical control systems, a machining program is stored in a memory and then, when necessary, numerical control information in the program is read out successively from the memory to permit the execution of a numerical control process in accordance with the instructions.
Magnetic bubble memory units have recently found use in such memories and they exhibit a number of advantages such as their large capacity and low cost, and the fact that they are non-volatile in nature. One disadvantage, however, is a comparatively long access time. Specifically, a magnetic bubble memory is capable of reading and writing data in page units, such as 64-byte units, but has a comparatively long access time of several milliseconds per page. As a result, a considerable amount of time is required for an editing process consisting of deleting a portion of the machining program stored in the magnetic bubble memory, revising the data, and adding new numerical control information. In particular, a fairly long period of time is required to alter numerical control information over a plurality of access units (page units in the case of a magnetic bubble memory) at one time. Problems occur in dealing with interruptions in power that may arise during this extended period of time.
An interruption in power during an editing process will mutilate the data stored in a memory if the interruption should occur in the midst of a write operation. For this reason a method which relies upon a parity check or the like is adopted to enable detection of the power interruption after the power has been restored. An interruption in power can, however, occur at other times, such as when the memory is not being accessed during, for example, the preparation of altered data which is to be written. If an interruption in power occurs at such time, a portion of the stored data which is to be rewritten will be data after the editing and rewriting, and the remaining portion will be data prior to the editing and rewriting, so that the total data will be meaningless. Despite this occurrence, however, the power interruption will go undetected with the method that utilizes a parity check or similar technique. This can lead to a machining operation which is based on an incorrectly modified machining program, the end result being a wasted workpiece and a decline in machining efficiency. In view of the foregoing problems there is a need for a method and system which permit the detection of an interruption in power, even if this occurs during the rewriting of stored data in a non-volatile memory, after the power has been restored.